findmnt: allow SOURCES field even without '--fstab'

findmnt: allow SOURCES field even without '--fstab'

The SOURCES output field was fully enabled only when "--fstab" is
enabled too. This patch enables SOURCES even without "--fstab".

$ ./findmnt -o TARGET,SOURCES
TARGET                                        SOURCES
/                                             /dev/sdg3
├─/mnt/btrfs-raid1                            /dev/sdb2
│                                             /dev/sde2
│                                             /dev/sda2
└─/tmp/t                                      /dev/mapper/pool-v2
                                              /dev/mapper/pool-v1

If '--fstab' is passed, and in fstab the filesystem is referred by UUID,
only one UUID for filesystem is showed:

$ ./findmnt -o TARGET,SOURCES --fstab
TARGET                         SOURCES
/                              UUID=d7a06504-cc14-435d-a5df-674da09c2894
/mnt/btrfs-raid1               UUID=b39b0b27-ff80-4cb4-bf48-0be939ff0788

However if you pass '-e', all the devices are showed

$ ./findmnt -o TARGET,SOURCES --fstab -e
TARGET                         SOURCES
/                              /dev/sdg3
/mnt/btrfs-raid1               /dev/sdb2
                               /dev/sde2
                               /dev/sda2

[kzak@redhat.com: - support all tags (not only LABEL or UUID),
                  - fix small memory leak,
                  - simplify code]

Signed-off-by: Goffredo Baroncelli <kreijack@inwind.it>
Signed-off-by: Karel Zak <kzak@redhat.com>